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Reference Dose (RfD) is an estimate of a daily exposure to a chemical that is likelyto be withoutan appreciable risk of harmful effects during a lifetime of exposure. It was developed by USEPAand is expressed in units of milligrams of contaminant per kilogram of body weight per day(mg/kg/day).

Minimal Risk Level (MRL) is an estimate of daily human exposure to a chemical that is likely tobe without an appreciable risk of harmful non-carcinogenic effects over a specified duration ofexposure. It does not protect hypersensitive individuals. MRLs were developed by ATSDRandare expressed in mg/kg/day.

Cancer Risk Evaluation Guides (CREGs) are estimated contaminant concentrations based onone excess cancer in a million individuals exposed to a chemical over a lifetime (70 years). Theseare very conservative values designed to protect sensitive members of the population.

Reference Dose Media Evaluation Guides (RMEGs) are estimates of a daily oral exposure to achemical that is unlikely to produce any non-cancerous adverse health effects over a lifetime. Theyare based on USEPA reference doses (RfDs) and are conservative values designed to protectsensitive members of the population.

Environmental Media Evaluation Guides (EMEGs) are comparison values developed byATSDR for chemicals that are relatively toxic, frequently encountered at National Priority List(NPL) sites, and present a potential for human exposure. They are derived to protect the mostsensitive members of the population (e.g., children), and are not cut-off levels, but rathercomparison values. They do not consider carcinogenic effects, chemical interaction, multiple routeof exposure, or other media-specific routes of exposure. They are very conservative concentrationvalues designed to protect the public.

Maximum Contaminant Level(MCL) is the maximum permissible level of a contaminant inwater which is delivered to any user of a public water supply that is protective of adverse humanhealth effects.

Secondary Maximum Contaminant Level (SMCL) is the maximum permissiblelevel of acontaminant in drinking water that do not affect aesthetic qualities of the water such as odor, taste,and color, and are related to public acceptance. These levels are not federally enforced.